1. Field of the Invention
This invention relates to an optical recording medium, particularly to an optical information recording medium for recording layer comprising at least one asymmetric trimethine-cyanine dye with electron-withdrawing substituted group.
2. Description of the Related Art
An optical recording medium capable of once recording by a laser ray has been known. For the structure of the CD-R information recording medium, a recording layer comprising an organic dye, a reflection layer and a protective layer are laminated on a transparent substrate by order. The recording information on the disc is performed by irradiating a laser at a wavelength of 780 nm to 830 nm to cause exothermic deformation of the recording layer. The information is usually reproduced by the same wavelength of laser ray, and the reflecting difference between the recorded area and unrecorded area is detected for the recording layer.
It is known that a laser ray with a shorter wavelength has a reduced light spot. Therefore, studies have being made to develop the optical disc capable of recording and reproducing by a laser beam of a wavelength of 620 nm to 690 nm to increase the recording density. Recordable digital video (DVD-R), which is capable of recording and reading in high density, is propagated as new media of the next generation. Since the wavelength zone for performing the recording and reproducing in DVD-R differs from that of the CD-R, the criteria for selecting the recording material to be employed for the CD-R cannot be applied to the DVD-R. Hence, new selection criteria are required to be established from a viewpoint peculiar to the DVD-R.
Trimethine-based cyanine dye is popular for DVD-R recording layer. U.S. Pat. No. 5,976,658 and U.S. Pat. No. 6,291,045 described the use of trimethine-based cyanine dye having symmetry or asymmetry indolenine structure for DVD-R. In U.S. Pat. No. 6,291,045, the absorbance ratio of the second peak to the main peak for the recording layer is disclosed to be less than 0.8 by using a visible ultraviolet spectrometer. However, the dye material should be selected carefully, while the writing wavelength of laser beam of DVD-R machines is changed from 635 nm to 650 nm (or further). Because the wavelength of laser power is getting longer, the dye absorbance of film state for 650 nm should be increased a little more. The dye, which is suitable for writing wavelength of 635 nm laser beam, is not performed well while the writing wavelength of laser beam is 650 nm. We suggest that the maximum absorption wavelength of the material with solution is in the range of 575 nm to 590 nm.
The first object of the present invention is to provide an optical information recording medium with a recording layer, which comprises at least one asymmetric trimethine-cyanine dye with electron-withdrawing substituted group.
The second object of the present invention is to provide asymmetric trimethine-cyanine dye with a counter anion, which improves the solubility of the asymmetric trimethine-cyanine dye in the coating solvent.
The third object of the present invention is to provide an optical information recording medium with improved write/read characteristics at the writing wavelength of 650 nm or further.
The invention provides an optical recording material With the structure represented by the formula [1], [2], [3], [4]. 
The xe2x80x9cEWGxe2x80x9d represents any electron-withdrawing substituted group such as a cyano, or a nitro group. The xe2x80x9cR1xe2x80x9d and xe2x80x9cR2xe2x80x9d are the same or different from each other and each is one of an individually substituted or unsubstituted alkyl, phenyl, benzyl, alkylphenyl and phenoxyalkyl group. The xe2x80x9cR3xe2x80x9d and xe2x80x9cR4xe2x80x9d are the same or different from each other and each is a are hydrogen atom, halogen atom, alkoxy, hydroxyl and alkyl group. The xe2x80x9cYxe2x80x9d is a halogen atom or an alkyl group, and xe2x80x9cXxe2x88x92xe2x80x9d is an anion selected from the group consisting of Fxe2x88x92, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, ClO4xe2x88x92, BF4xe2x88x92, PF6xe2x88x92, SbF6xe2x88x92, OTsxe2x88x92, OMsxe2x88x92, SCNxe2x88x92, alkylsulfonate and alkylcarboxylate. The material has a maximum absorption wavelength from 575 nm to 590 nm in acetonitrile solution. It is used as a recording material for DVD-R machine with the laser wavelength from 620 nm to 690 nm.
The material used in the dye layer also selectively comprises the trimethine-cyanine dye of formula [5]. 
The xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d represent any one of the following formulas [6], [7], [8], [9]. 
The xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d may be the same or different from each other. The xe2x80x9cDxe2x80x9d may be substituted or unsubstituted alkyl, carboxyl, alkoxycarbonyl, alkylcarboxyl, alkoxyl, alklhydroxyl, aralkyl, alkenyl, alkylamide, alkylamino, alkylsufonamide, alkylcarbamoyl, alkylsulfamoyl, hydroxyl, halogen atom, hydrogen atom, alkylalkoxyl, alkylhalide, alkylsulfonyl, alkylcarboxyl or alkylsulfonyl which are bonded to a metallic ion or alkyl, phenyl, benzyl, alkylphenyl or phenoxyalkyl group. The xe2x80x9cpxe2x80x9d represents an integral or more.
The xe2x80x9cR5xe2x80x9d and xe2x80x9cR6xe2x80x9d may be the same or different from each other and are individually substituted or unsubstituted alkyl, phenyl, benzyl, alkylphenyl or phenoxyalkyl group. The xe2x80x9cZxe2x80x9d is hydrogen atom, halogen atom or alkyl group, and xe2x80x9cXxe2x88x92xe2x80x9dis an anion selected from the group consisting of Fxe2x88x92, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, ClO4xe2x88x92, BF4xe2x88x92, PF6xe2x88x92, SbF6xe2x88x92, OTsxe2x88x92, OMsxe2x88x92, SCNxe2x88x92, alkylsulfonate, akylcarboxylate.
The manufacture of the optical information recording medium according to this invention can be performed as follows.
(1) A solution contains a cyanine dye represented by general formula [1], [2], [3], [4] or a solution contains a cyanine dye represented by general formula [1], [2], [3], [4] mixed with the dye of formula [5], which is less than 100% by weight. The solution prepared above may be added in some kind of singlet oxygen quencher such as a metal complex, a light absorbent, a radical scavenger, etc. The solvent used in the dye solution may be selected from fluorinated alcohol, diaceton alcohol, methylethyl ketone, methanol, toluene, cyclohexanone, acetylacetone, dioxane. The mixing ratio of the cyanine dye should be 0.5% to 50% by weight.
(2) The material for the substrate may be employed by glass, epoxy resin, methacryl resin, polycarbonate, polyester resin, polyvinyl chloride resin, polyolefin resin. The substrate may be provided with tracking grooves or pits.
(3) The dye solution is provided on a substrate by means of a spin-coating method.
(4) The optical information recording medium may further include a reflection layer in addition to the recording layer. The reflection layer, a film of high reflectivity, can be formed by the vapor-deposition or sputtering a metal, such as Gold, Aluminum, Silver, Cupper, Platinum or alloy.
(5) The reflection layer may be provided with protective layer. It is formed for the purpose of protecting or improving the optical information recording medium, and can be formed by coating a solution of a radiation cure type resin on a given surface and then by radiation-curing the coater layer.
(6) The optical information recording medium may be a single-sided recording medium obtained by laminating a single-side substrate and a non-grooved substrate with an adhesion. The single-sided optical recording medium being prepared by process (1) to (5).
(7) The optical information recording medium may be a double-sided recording medium obtained by laminating two single-sided optical recording medium with an adhesion. The single-sided optical recording medium being prepared by process (1) to (5).
The invention will be further explained in detail for the following preferred embodiments.
Asymmetric dyes, which are trimethine-cyanine dyes substituted with different structure comprising electron-withdrawing group, could be synthesized by different methods suggested in J. Org. Chem. 1995, 60, 2411 and U.S. Pat. No. 6,306,478 (Scheme 1, 2, 3).
For example, the xe2x80x9cEWGxe2x80x9d is a nitro group, the xe2x80x9cR3xe2x80x9d and xe2x80x9cYxe2x80x9d are hydrogen group by the formula [1]. The formula [11] could not be obtained easily through scheme 1, because formula [10] is low reactivity and specific reactive condition needs to undergo in a sealed tube. 
Even if formula [11] is obtained, the reaction will not undergo path 1 according to scheme 2. In stead, the reaction will follow path 2. And formula [13] is obtained at the refluxing condition with acetic anhydride solvent. 
In scheme 3, the compound of formula [1] will be obtained but yield is low. 
So we designed a modified method in scheme 4, the reactive condition is under atmosphere and formula [1] is obtained with high yield. From the following reaction figure of scheme 4, formula [15] and formula [11] could be obtained easily with high yield by using well know alkylation and nitration processes. Then, formula [11] is dissolved in alcohol solvent and is mixed with N,N-diphenylformamidine under refluxing. Next, formula [16] is obtained with ideal quality and yield. Finally, formula [16] and formula [13] are treated with basic solvent, stirred at room temperature, and formula [1] is obtained in high yield. 
Examples of the compounds by the formula of [1], [2], [3], [4], which are synthesized by the similar method in scheme 4, are given below.
A polycarbonate substrate is provided with a track pitch 0.74 xcexcm and only wobble signal (pre-pit may also be included) is employed.
A solution contains trimethine-cyanine dyes of formula [1], [2], [3], [4] with 0.1% to 50% by weight or contains the mixture of trimethine-cyanine dyes of formula [1], [2], [3], [4] and one of formula [5], which is included less than 100% weight. The solution prepared above may further comprises some kind of singlet oxygen quencher such as a metal complex, a light absorbent, a radical scavenger, etc. The solvent used in the dye solution may be selected from fluorinated alcohol, diaceton alcohol, methylethyl ketone, methanol, toluene, cyclohexanone, acetylacetone, dioxane. Then, the substrate is spin-coated with the dye solution at a rotational frequency of 3000 rpm, and then is dried at 70xc2x0 C. for 20 minutes to form a recording layer consisting of a photosensitive dye film with a thickness of 100 nm.
There after, a reflection layer of Au or Ag is deposited on the recording layer by sputtering. A protective layer comprising an ultraviolet-curing resin is further spin-coated on the reflection layer. Then a pair of the optical discs thus obtained is superimposed via an adhesive layer comprising an ultraviolet-curing resin, and thereby a laminated-disc is obtained.
Next, a laser beam of 650 nm or further is irradiated to the optical discs to form recording signals with improved characteristics, such as modulation amplitude, jitter and wide power margin.